2. Does it slow down my store?
When this module is active it checks the module config every time a page is loaded. If you add too many IP addresses into the module then yes, it could possibly slow down your store.
However, this module is made for temporary IP blocking and the list should always be kept short (a few dozen at the most.) If it gets too big, contact your host so they can add the IPs to a bad IP list that's on the server level rather than the store level.